A subsea well assembly undergoes several installation procedures, including drilling, completion, and production installation procedures. The subsea well assembly will have an outer or low pressure wellhead housing secured to a string of conductor pipe which extends some short depth into the well. An inner or high pressure wellhead housing lands in the outer wellhead housing. A drilling riser connector is connected to the inner high pressure wellhead housing. A casing hanger is installed in the wellhead housing, and the high pressure wellhead housing is secured to an outer string of casing, which extends through the conductor pipe to a deeper depth into the well, after which the casing is cemented. Depending on the particular conditions of the geology above the target zone, one or more additional casing strings will extend through the outer string of casing to increasing depths in the well until the well is to the final depth.
When the drilling operations are finished, the drilling riser connector is removed from the inner high pressure wellhead housing. In one type of subsea well, a tubing hanger is installed in the wellhead housing inside the casing, and a tubing string extends into the well for production. Then a production connector body carrying a production tree lands on the high pressure wellhead housing in communication with the tubing hanger.
In prior versions of subsea wells, a metal seal ring seals between the high pressure wellhead housing and the production tree at the rim of the high pressure wellhead housing. This results in a fairly large diameter seal ring, which can be difficult to achieve at high pressure ratios.